Manufacturing rails: additive, subtractive or conventional? 

Splints are an indispensable part of modern dentistry - whether in the treatment of bruxism or craniomandibular dysfunctions. But which manufacturing method delivers the best results? A team of researchers from LMU Munich has investigated the mechanical and physical properties of splints that were manufactured additively (3D printing), subtractively (milling) or conventionally. 

The results of the study presented here show exciting differences: While milled and conventionally manufactured splints have the edge in terms of strength, 3D printing offers advantages in terms of material efficiency and flexibility. However, the choice of material plays a central role, especially under load - and not only when dry, but also after water absorption (oral environment).

Mechanical and physical properties of additively, subtractively and conventionally manufactured rails

Splints in dentistry have a wide range of applications. They are most commonly used to treat bruxism and craniomandibular dysfunctions. Conventionally manufactured injected splints or deep-drawn splints have disadvantages such as residual monomer content, polymerization shrinkage, susceptibility to errors due to mixing, high time expenditure and relatively high laboratory costs. CAD/CAM technologies – subtractive milling or additive 3D printing – offer advantages such as easy access to patient-specific data and rapid production in the event of breakage or loss. 

Subtractive milling enables a better fit due to the high conversion rate of the double bonds in polymer-based discs. However, a maximum of two rails can be placed in one disc, which leads to high material consumption and higher costs. Consequently, additive manufacturing is gaining importance as it enables the printing of complex geometries, the placement of multiple restorations on the build platform and a reduction in material consumption. 

Current studies show that injected and milled splint materials have higher mechanical properties than 3D printed materials. The pressure angle and post-polymerization in additive manufacturing influence these properties. 

Since rails can break under load, their mechanical properties are important. Hardness is a measure of the elastic-plastic behavior of a material, while flexural strength and elastic modulus measure fracture strength and stiffness. According to current studies, the mechanical properties of 3D printed materials deteriorate after water absorption. This highlights the need to further investigate the interactions between mechanical properties, water absorption and solubility. This study aimed to investigate the influence of material and artificial aging on mechanical and physical properties. The null hypotheses tested were that material and aging have no influence on the properties studied.

material and methods

A total of 1140 test specimens were produced in three different geometries (Figure 1).

• five additive (GR-22 flex [abbreviated to: aPF], GR-10 guide [aPG], ProArt Print Splint clear [aPP], V-Print Splint [aVS], V-Print Splint comfort [aVC]), five milled (BioniCut [bBC], EldyPlus [bEP], ProArt CAD Splint clear [bPC], Temp Premium Flexible [bTP], Thermeo [bTH]) and 
• two injected (PalaXPress clear [cPP], Pro Base Cold [cPB]) materials.

After 24 hours of water storage (37 °C), 90 days of water storage (37 °C) and after cyclic thermal cycling (5000 cycles, 5/55 °C), flexural strength [FS], elastic modulus [E], Martens hardness [HM], water absorption [w_sp], water solubility [w_sl] and conversion rate [DC]. The collected data were statistically analyzed using Kolmogorov-Smirnov, Kruskal-Wallis, Mann-Whitney U test and Spearman correlation. Statistical significance was assumed when p-values ​​were < 0,05.

Results

The composition of the material influences the mechanical and physical properties, with significant differences observed within the 3D printed and milled groups. 

During the FS measurement, only specimens made of aVS, bPC, cPP and cPB broke. The milled PMMA and polycarbonate materials showed the highest FS values ​​after all aging regimes. The printed aVS showed mechanical properties comparable to those of the milled materials and was the only printed material to show FS and E values ​​above 65 MPa and 2 GPa both initially and after all aging regimes. The additively manufactured specimens showed higher w_sp– and w_slvalues ​​compared to the milled and conventional groups. The 3D printed materials – except aVC and aPF – and the injected materials were more susceptible to aging in terms of FS than the materials of the milled group. Aging did not significantly reduce hardness; except for aPP.

Conclusion

Given the lower E and HM values ​​of all 3D printed materials – with the exception of aVS – their clinical use in patients with severe bruxism should be viewed critically, as the materials must withstand high chewing loads of up to 770 N. The use of these soft splint materials in patients with craniomandibular dysfunction should be investigated in in vivo studies. The clinical indication of the splint should therefore be the decisive criterion for the choice of material.